44 Notes on Barometer Construction. 



expired gases are charged with organic contaminations ; a 

 purely mechanical air pressure, as that supplied by a com- 

 pressed indiarubber ball or condensing syringe, is free from 

 this objection. 



If sealed junctions are necessary for the construction of 

 the barometer, these are not satisfactorily effected by press- 

 ing merely softened glass surfaces together ; the glass tube 

 ends to be joined must be well melted in the flame, then 

 joined, and the joint must be retained in the molten con- 

 dition in the flame until the whole of the softened portion 

 has become identified into one homogeneous mass. Atten- 

 tion to the necessity of annealing such work as far as prac- 

 ticable will influence its durability. The air-driven gas 

 flame used should, when lead glass is the subject in hand, be 

 sufficiently oxygenated to prevent reduction of lead oxide 

 to the metallic state and consequent blackening of the tube. 

 One final remark, especially addressed to beginners in the 

 work, is the advice to mark out in pencil on a smooth 

 pine board the dimensions of the piece to be made at the 

 lamp ; this outline is used as a gauge with which to try the 

 dimensions and angles of the piece, by juxtaposition, as it 

 proceeds. 



So much concerning the glass tube^ whether for cistern or 

 syphon barometer. Let us in the next place pay a few minutes' 

 attention to the mercury. The mercury must be pure and 

 dry, and free from all superficially adherent particles. When 

 we allow a beam of sunlight to fall through a shutter hole 

 into an otherwise dark apartment, we see that the air is 

 permeated throughout with minute floating solid particles — 

 motes which gyrate and eddy with every motion of the air, 

 and which gravitate so slowly that in very few positions 

 indeed is the air free from them. Among these particles are 

 the germs which insinuate themselves between the lenses of 

 telescopes, start into vegetative life_, and feed on the glass 

 surfaces, deadening them, just as the familiar lichen 

 establishes itself upon and assists the decay of the hard 

 surfaces of igneous rocks. I refer to these bodies with the 

 object of calling your attention to the great necessity of 

 employing the utmost care in the construction of glass 

 instruments of the nature of the barometer, and the great 

 difficulty of effecting absolute cleanliness of the glass inner 

 surfaces, and the mercury to be employed, even when very 

 great precautions are taken. Fortunately it is not difficult 



